There are varieties of helmet designs. Each of them is suitable for a specific group of activities. In general, a helmet structure consists of a rigid outer shell, an impact absorbing liner, fitting padding, and a retention system. The outer shell's role is to avoid any penetration to the interior of the helmet as well as to distribute the impact load uniformly over the liner. The liner's function is to absorb the energy of impact. Spreading out the impact load increases the energy absorption capacity of the liner. When an impact takes place, the outer shell stops instantly. However, inside the helmet the head keeps moving until it collides with the liner. The liner's role is to bring the head to a “gentle” stop. In other words, the liner's major role is to reduce the absolute value of translational acceleration of the wearer's head. A stiffer liner results in more impact load on the brain during an accident, while a softer liner transmits less impact to the brain.
Present helmets focus primarily on reducing translational acceleration upon impact. However, rotational acceleration may also prove to be injurious. In fact, recent studies have shown that rotational acceleration is a principal cause of head injury. It has been proven that without exceeding the thresholds for head injury with respect to translational acceleration, the rotational acceleration can reach injurious magnitudes. Therefore, it is necessary to improve helmet design to not only reduce the translational acceleration, but also to reduce rotational acceleration.
WO 2010/151631 A1 defines a protective headgear with an outer shell rotatable relative to an inner shell regardless of the direction of an impact load via an intermediate layer disposed between the inner shell and outer shell. The intermediate layers are comprised of a substantially isotropic yielding material that deforms continuously and nonlinearly in the tangential or shear direction. The layer deforms during an impact to allow rotation of the inner shell relative to the outer shell to reduce rotational acceleration of the user's head.
WO 03005844 A1 discloses a protective headgear, which has a feature to reduce rotational acceleration of a user's head during occurrence of an impact. The invention comprises a shell of typical headgear and a single elastomeric outer membrane (the single layer may comprise composite or laminate material), which overlies the outwardly facing surface of the headgear shell. The single elastomeric outer membrane comprises closed cell plasticized polyvinyl chloride, polyethylene, and ethylene-vinyl acetate co-polymers. In addition, a lubricant material may be provided between the shell and outer membrane. As the outer membrane experiences friction force during an impact with an obstacle, the received force causes the outer membrane to move relative to the headgear shell. Since the lubricant material minimizes the friction between the shell and outer membrane, the headgear slides with respect to the impact surface and the membrane. The invention simulates the protective movement of the human scalp relative to the skull. The outer membrane is designed to mimic the scalp, which is not attached firmly to skull, but is instead free to move a limited distance relative to the skull. This mechanism is described as mitigating the injurious effect of the tangential component of the impact force during an impact, and reduces the rotational (i.e., angular) acceleration of the wearer's head.
WO09019667A introduces a coating for a cap of a helmet, wherein the coating comprises a first layer of resilient material having a first surface arranged to be secured to the cap with an adhesive means and a second surface opposite the first surface, and a coating layer, secured with the adhesive means to the second surface of the layer of resilient material. This improves helmet capabilities for safe protection and the coating is easily adaptable on different sizes of helmets.
In general, available helmets provide no protection against rotational accelerations. Therefore, there is a need for an impact diverting mechanism as an effective add-on layer that can be easily installed on the outer shell of almost any type of safety equipment such as helmet by the users or manufacturers.